I note you didn't comment on the suggestions to use more fat. My personal preference is to use a minimum of 25% fat, and when I say minimum I mean just that. Tradtionally sausages in the UK had up to 35 or even 50% fat!! I hesitate to disagree with the far more experienced sausagemakers here, but I also prefer to use more salt.
What I don't think anyone's directly said is that you should ALWAYS taste your mix before stuffing into skins. Do all the mincing, seasoning and mixing that you;re going to do, put the sausagemeat into the fridge, after taking out about an ounce. Shape that into a thin patty and fry it on both sides in a non-stick pan for about 5 mins a side. Allow to cool slightly and then taste - carefully and self critically. Are you happy with the seasoning? Is it succulent? This gives you a chance to correct the seasoning. If it's not succulent enough then you'te brobably lacking fat. If that's the case, mince some more and mix it in, and test again.